The present invention relates to integrated circuit structures and fabrication methods, and particularly to the integration of fluorine containing oxides in standard interconnect metallization.
Due to the many desired properties of conductor structures in integrated circuits, the number of metals which are used is very small. Much work is being done on the use of copper for metallization, but currently, the majority of circuits use aluminum (Tungsten is also used in some cases.) These two metals will typically have adhesion/barrier layers between them and the dielectric layer, so that typical metallizations use a sandwich of, e.g. Ti/TiN/Al/TiN or Ti/TiN/W.
There are also many constraints on the dielectrics which are used in integrated circuits. Typical dielectrics are doped CVD SiO2, TEOS-based SiO2 films, PECVD silicon nitrides, bias-sputtered SiO2, polyimides, and spin-on glasses. For further information, see S. Wolf, Silicon Processing for the VLSI Era: Volume 2xe2x88x92Process Integration (1990), which is hereby incorporated by reference.
Fluoro-Silicate Glass (FSG) in Interconnect Structures
It is desirable to, be able to integrate fluorine-containing dielectrics, such as fluoro-silicate glass (FSG), into the standard interconnect metallization scheme, as these materials have a low coefficient of permittivity (k). However, use of FSGs in interconnect formation has shown problems of metal xe2x80x9ccorrosionxe2x80x9d and metal delamination at high fluorine concentrations. This is seen as a delamination of the metal film from the dielectric and appears when the subsequent metallization layer is deposited (e.g., metal-2 will exhibit delamination from the underlying FSG only after the metal-3 layer is deposited.
It has been observed that this delamination is seen only when a metal such as titanium is deposited over fluorinated oxides and not when FSG is deposited over the metal. Additionally, peeling is not observed when TiN is deposited over fluorinated oxide, or when FSG is deposited over TiN.
FSG films on Si wafers used for film measurements have been found to be stable even with F content  greater than 16% at (Kxe2x89xa63.0). Various test methods, including MOSCAP measurements and Thermal Desorption Spectroscopy, which have been performed on these films at various stagesxe2x80x94as deposited, after temperature cycling at 450-500 C for 30 minutes, after moisture-stressing and over timexe2x80x94indicate that the films by themselves are stable. However, integration of these FSG""s pose challenges due to the above-mentioned interactions with the metal stack during integration.
The present inventors have realized that the adhesion problems which occur when a layer of metal is deposited on a fluorine-containing dielectric are due to a reaction between available surface fluorine atoms and the lowest deposited layer of metal atoms. When the activated metal, such as titanium, strikes the FSG, they combine to form fluorides, such as TiF4. TiF4, as an example, will sublime around 250-270 degrees C, which is far below the current metallization temperatures of 400 or greater. This, then, is the cause of the drastically reduced adhesion between a metallic layer and the fluorine-containing dielectric which have been observed, and which is illustrated in FIG. 3. Here metal layer 80 was deposited over fluorine-containing dielectric 50, with the resultant formation of fluoride layer 100. At the left of the figure, the volatilization of the fluoride layer 100 allows the delamination of metal 80. The reaction shown does not occur when the metal is deposited first, since it can already have a passivating oxidation layer, which appears to prevent further reaction. While the use of thick undoped oxide over the fluorine-containing dielectric may work, this would reduce the advantage of a low-k dielectric. The present application discloses forming a passivation layer over a fluorine-containing dielectric to prevent reaction of the fluorine with metal. This passivation may take several forms:
a.) a very thin layer of aluminum (e.g., 30 nm) can be deposited over a blanket dielectric, with the formation of Al203 on its surface;
b.) a layer of titanium nitride, or alternatively tungsten nitride (WN) or tantalum nitride (TaN), can be used to line the sidewalls of contacts or vias;
c.) a layer of silicon nitrides can be formed on the surface of the FSG by treatment with an N2 plasma.
These passivation layers can be used to prevent the reaction of a fluorine-containing dielectric with titanium, tantalum, tungsten, molybdenum, etc., wherever the metal fluorides are volatile at subsequent processing temperatures.
Advantages of the disclosed methods and structures include:
prevents delamination of metallization from dielectric;
at most, two additional steps are necessary;
uses conventional equipment and processes.